1. Field of the Invention
The invention relates to a decoder for use in a digital transmission system for transmitting a wide-band digital audio signal.
The invention further relates to a receiver for use in such transmission system, and to a method for decoding the encoded wideband digital audio signal.
2. Description of the Related Art
A transmission system of the type defined in the opening paragraphs is known from the article “The Critical Band Coder—Digital Encoding of Speech signals based on the Percentual requirements of the Auditory System” by M. E. Krasner, in Proc. IEEE ICASSP 80, Vol. I, pp. 327–331, Apr. 9–11, 1980. This article relates to a transmission system in which the transmitter employs a sub-band coding system and the receiver employs a corresponding sub-band decoding system, but the invention is not limited to such a coding system, as will become apparent hereinafter.
In the system known from this publication, the speech signal band is divided into a plurality of sub-bands having bandwidths approximately corresponding to the bandwidths of the critical bands of the human ear in the respective frequency ranges (cf. FIG. 2 in the article of Krasner). This division has been selected because, on the ground of psycho-acoustic experiments, it is foreseeable that the quantization noise in such a sub-band will 10 be masked to an optimum extent by the signals in this sub-band if, in the quantization, allowance is made for the noise-masking curve of the human ear (this curve giving the threshold value for noise masking in a critical band by a single tone in the center of the critical band, cf. FIG. 3 in the article by Krasner).
It should, however, be noted that the invention is not restricted to an encoding into sub-band signals. It is equally well possible to apply transform coding in the encoder, a transform coding being described in the publication “Low bit-rate coding of high-quality audio signals. An introduction to the MASCAM system” by G. Theile et al., in EBU Technical Review, No. 230 (August 1988).
In the case of a high-quality digital music signal, which, in conformity with the Compact Disc Standard, is represented by 16 bits per signal sample in the case of a sample frequency of 1/T=44.1 kHz, it is found that with a suitably selected bandwidth and a suitably selected quantization for the respective sub-bands, the use of this known sub-band coding system yields quantized output signals of the coder which can be represented by an average number of approximately 2.5 bits per signal sample, the quality of the replica of the music signal not differing perceptibly from that of the original music signal in substantially all passages of substantially all kinds of music signals.
The sub-bands need not necessarily correspond to the bandwidths of the critical bands of the human ear. Alternatively, the sub-bands may have other bandwidths, for example, they may all have the same bandwidth, provided that allowance is made for this in determining the masking threshold.